1. Field of the Invention
The present invention generally relates to the layer structure of conductive patterns suitable for multi-layer substrates, and more particularly to a conductive pattern layer structure comprising a first thin film formed on an insulating layer made of, for example, polyimide, a second thin film formed on the first thin film, and a conductive layer formed on the second thin film. Further, the present invention is concerned with a method of producing such a conductive pattern layer structure.
2. Description of the Related Art
A multi-layer substrate used in electronic devices is made of a conductive pattern formed with a copper layer, and an insulating layer made of polyimide and formed on the conductive pattern.
Recently, there has been considerable activity in the development of a fine conductive pattern. In a fine conductive pattern, there is a possibility that copper may be diffused into the polyimide layer and hence a dielectric layer may be formed around the conductive pattern. The above dielectric layer affects the propagation speed of a signal propagated through the conductive pattern. Further, a space may be formed between the conductive layer and the insulating layer, so that the insulating breakdown voltage deteriorates. Hence, it is desired to prevent copper contained in the conductive pattern from being diffused into the polyimide layer.
FIG. 1 is a cross-sectional view of a related conductive pattern layer structure, and FIGS. 2A through 2E are cross-sectional views of a method of producing the conductive pattern layer structure shown in FIG. 1.
As shown in FIG. 1, a conductive pattern 12 is configured as follows. A first patterned thin film 3 made of chromium is formed on a predetermined surface portion 1A of an insulating member 1 made of polyimide, and a second thin film 4 having the same pattern as the first thin film 3 is formed on the first thin film 3. A conductive layer 5 made of copper is formed on the second thin film 4, and a Cr layer 13 is formed on an upper surface 5B of the conductive pattern 5.
The conductive pattern 12 shown in FIG. 1 is produced by the following process. First of all, as shown in FIG. 2A, the first thin film 3 made of Cr is formed to a thickness of 1000-2000 .ANG. on the predetermined surface portion 1A of the insulating member 1 by sputtering. Next, the second thin film 4 is formed to a thickness of 0.2-0.5 .mu.m on the first thin film 3 by sputtering. In this manner, the second thin film 4 is strongly adhered to the insulating member 1 via the thin film 3. A resist layer 10 is formed on the second thin film 4, and is then developed so that the resist layer 10 is patterned. The conductive layer 5 is formed to a thickness of 4-6 .mu.m on the second thin film 4 appearing through the patterned resist layer 10 by copper plating. Then the resist layer 10 is removed, and a test process for validating the conductive pattern 5 may be performed.
As shown in FIG. 2B, another resist layer 10' is formed on the second thin film 4 and tile conductive layer 5, and the Cr layer 13 is formed to a thickness of 1000-2000 .ANG. on the conductive layer 5 and the resist layer 10' by sputtering. Then, as shown in FIG. 2C, the resist layer 10' and the part of the Cr layer 13 formed on the upper surface of the resist layer 10' are removed by a lift-off process, so that only the part of the Cr layer 13 formed on the conductive layer 5 remains. Then, as shown in FIG. 2D, a resist layer 14 is formed on the Cr layer 13. As shown in FIG. 2E, the first and second thin films 3 and 4 are partially removed by a wet etching process in which the resist layer 14 functions as a mask. In this manner, the first and the second thin films 3 and 4 are patterned to have the same pattern as the conductive layer 5. Finally, the resist layer 14 is removed, and thus the conductive pattern 12 is completed.
When an insulating layer made of polyimide is stacked on the conductive pattern 12 so that it covers the pattern 12, the insulating layer is formed on the Cr layer 13. Hence, it is possible to prevent copper from being diffused into the polyimide insulating layer via the upper surface of the conductive layer 5.
However, the above-mentioned related art has the following disadvantages. While the first and second thin films 3 and 4 are being removed by the wet etching process, the sidewalls of the conductive layer 5 and the sidewalls of the first and second thin films 3 and 4 are etched by side-etching, as indicated by broken lines shown in FIG. 1. Hence the widths of the first and second thin films 3 and 4 and the conductive layer 5 are less than a predetermined width B. Further, the upper surface of the conductive layer 5 is covered by the Cr layer 13, while the sidewalls of the conductive layer 5 are exposed. Hence, copper contained in the conductive layer 5 is diffused into another insulating layer surrounding the conductive layer 5 via the sidewalls of the conductive layer.